Off-grid solar charge controllers

[Jason]

Sorry I didn't get to say this sooner, but it's a pleasure having you here at Solar Panel Talk Jonathan. Your contribution is greatly appreciated.

[Sunking]

So, the advantage of MPPT controllers in warmer climates may not be worth the extra cost. If you have a 1200 watt system on a fixed tilt, it will put out around 60 amps peak output at 12 volts. The MPPT may give you less than a 10% gain or less than 120 watts advantage and only part of the time. The $300 extra that you pay for the MPPT controller could purchase you an additional 70 watts of PV panels which have a 25 year warranty. The MPPT controller only has a 2 year warranty (5 year extended warrant costs extra). I would go for the extra generating capacity and use the lower cost charge controller.

John first let me say WELCOME, you are a VALUED CONTRIBUTOR.

With that said i respectively disagree with your oppinion. A shunt (PWM) regulator by nature is 70% efficient, whre MPPT is 90% or more regardless of weather.

[Jonathan Cole]

a)
b) MPPT vs PWM Well, they are different creatures. one can down convert higher voltage to battery voltage, and reduce losses in wireing. PWM is brain dead simple, but if your panels are not voltage matched to your batteries, you can have a lot of losses. PWM, in hot weather, may not deliver enough voltage for a full charge, because PV panels have lower voltage when hot. MPPT is more complicated, but also more flexible.

Here is a bit more in depth info on charge controllers.


There are several approaches to charge control. These are Pulse Width Modulation (PWM), Maximum Power Point Tracking (MPPT), and in some cases power diversion when the batteries are full. See the descriptive technical articles in the Appendix.

You can simply connect the PV panels directly to the battery or inverter. The older charge controllers, simply turned the panels on or off according to the battery voltage and capacity. This is not advisable, however, due to over-voltage conditions, over-heating, shortened battery life and decreased battery capacity. The investment in a charge controller is well worth the savings in battery life and capacity.

As a result of the deficiencies of these older on-off devices, a class of charge controllers using Pulse Width Modulation (PWM) were developed that give an optimal charging regime for the deep cycle batteries used in solar applications. Highly efficient and very low cost per watt of controlled power, these have become very sophisticated and usually include volt, amp, and accumulating kwHr metering.

The Maximum Power Point Tracking (MPPT) charge controller differs in that it calculates the voltage at which the module is able to produce maximum power. The MPPT system then operates the PV modules to extract the full wattage, regardless of present battery voltage. A high efficiency DC-to-DC power converter converts the module voltage at the controller input to the appropriate battery voltage at the output.

With MPPT, if the whole system—wiring and all—was 100% efficient, a charge current increase of 42% would be achieved by harvesting module power that would have been left behind by a conventional controller. But nothing is 100% efficient and actual charge current increase will be somewhat lower as some power is lost in wiring, fuses, circuit breakers, and in the charge controller itself.

Actual charge current increases from MPPT controllers vary with operating conditions. The greater the difference between the PV module maximum power voltage and the battery voltage, the greater the charge current increase from an MPPT controller will be.

Cooler PV module cell temperatures tend to produce greater charge current increase. A highly discharged battery will also increase charge current since battery voltage is lower, and output to the battery during MPPT could be thought of as being “constant power.”

What can be expected of MPPT controllers in cool temperatures with typical battery conditions is a charge current increase of between 10–25%. Cooler temperatures and highly discharged batteries can produce increases in excess of 30%.

Customers in cold climates have reported charge current increases in excess of 40%. What this means is that in colder climates, current increase tends to be greatest when it is needed most; in cooler conditions when days are short, the sun is low on the horizon and batteries may be more highly discharged. In conditions where extra power is not available (highly charged battery and hot PV modules) a MPPT charge controller will perform as a conventional PWM type controller.

However, an important caveat is that MPPT charge controllers tend to be a lot more expensive than PWM controllers. For example, an Outback 60 amp MPPT charge controller with a 2 year warranty costs around $490.00 plus shipping. A Morningstar 60 amp PWM charge controller with a 5 year warranty costs $200 plus shipping.

So, the advantage of MPPT controllers in warmer climates may not be worth the extra cost. If you have a 1200 watt system on a fixed tilt, it will put out around 60 amps peak output at 12 volts. The MPPT may give you less than a 10% gain or less than 120 watts advantage and only part of the time. The $300 extra that you pay for the MPPT controller could purchase you an additional 70 watts of PV panels which have a 25 year warranty. The MPPT controller only has a 2 year warranty (5 year extended warrant costs extra). I would go for the extra generating capacity and use the lower cost charge controller.


_____________________________
Jonathan Cole, MBA
Author & Solar Energy Consultant

https://www.lightontheearth.org